1. Field of the Invention
The present invention relates to a process for the preparation of formyl valeric acid and adipic acid or esters thereof.
2. Description of Related Art
The current production of caprolactam and adipic acid is based on processes requiring a lot of energy and leads to release of greenhouse gases such as CO2 and NOx. Thus, there is a great incentive to replace these processes by other processes leading to less use of energy and in which the amount of greenhouse gases released is substantially less.
A known production route to caprolactam proceeds via 5-formylvaleric acid, which can be made from butadiene. WO94/26688 discloses a process to produce linear aldehydes such as 5-formylvaleric acid from an isomeric mixture of methyl pentenoates. The process of WO94/26688 starts with an isomerisation step in order to increase the amount of methyl-4-pentenoate, and proceeds with a non-isomerising hydroformylation step of the isomerised mixture of methyl pentenoates acids using a Rh catalyst to produce a mixture of 5-formyl valeric acid and remaining methylpentenoates consisting mainly methyl-3-pentenoate and some methyl-2-pentenoate. Next, the 5-formyl valeric acid is separated from the remaining methyl pentenoates and the remaining methyl pentenoates are fed back to the isomerisation step.
EP1251122-A1 relates to a process for the preparation of ε-caprolactam starting from butadiene, carbon monoxide, hydrogen and ammonia, by (1) carbonylating butadiene in the presence of an alkanol and a catalyst comprising palladium, a multidentate phosphine ligand and an acidic co-catalyst to produce alkyl-4-, alkyl-3- and alkyl-2-pentenoate; (1′) optionally isomerising the alkyl-3- and/or alkyl-2-pentenoate into alkyl-4-pentenoate; (2) hydroformylating the alkyl-4-, alkyl-3- and alkyl-2-pentenoate in the presence of a catalyst comprising rhodium and an organic phosphorous containing ligand to produce alkyl-5-formylvalerate; (3) reductively aminating alkyl-5-formylvalerate in the presence of a hydrogenation catalyst comprising ruthenium on a carrier to produce ε-caprolactam and ε-caprolactam precursors; and (4) optionally converting ε-caprolactam precursors at elevated temperature into ε-caprolactam.
A problem with the isomerisation of pentenoic acids (or esters) is that the equilibrium constant favours the formation of 3-pentenoic acid, and that relatively little 4 pentenoic acid is formed. Thus, a problem of WO94/26688 is that the recycle step is very inefficient since with every recycle step only a small fraction of the remaining 2 and 3 pentenoate esters is isomerized into 4-pentenoate ester and subsequently converted to methyl 5-formylvaleriate This means that in the process of WO94/26688 a substantial fraction consisting of 2 and 3 pentenoic acid esters is more or less perpetually recirculated. This is of course economically undesirable.
An option would be to simply discard any remaining 2 and 3 pentenoic esters but this would result in an unacceptable waste. An alternative solution suggested by WO94/26688 is to enrich the starting mixture in methyl 4-pentenoate by separating it from the other isomers using distillation. However, it is known in the art [ref] that due to their similarity it is very difficult to separate methyl 4-pentenoate from methyl 2- and 3-pentenoate. Moreover, this would mean extra cost due to in expensive distillation equipment.
It is an aim of the invention to provide a single process to make both formylvaleric acid and adipic acid or esters thereof from an isomeric mixture of pentenoic acids or esters. It is another aim to provide a process which allows for efficient use of an isomeric (alkyl) pentenoic acid mixture in making polyamide intermediates, which process is preferably simple and/or affords good yield and/or selectivity, and/or which results in little or no wasted side products.